You might not be losing weight because you are burning calories through cardio but eating back those calories without realizing it. Even healthy foods can add up if your portions are larger than you need. Try tracking your food intake for a few days and focus on maintaining a consistent calorie deficit rather than relying only on exercise.
One reason could be that you are building muscle while losing fat. If you combine cardio with strength training your body composition may improve even when the number on the scale does not change much. Pay attention to your waist measurements progress photos and how your clothes fit instead of judging your results only by body weight.
You might be doing the same cardio workout every day without changing your overall activity level. As your fitness improves the same workout can become easier and may burn fewer calories than you expect. Gradually increasing your walking time or workout intensity can help but you should also pay attention to your diet and recovery.
Your weight may fluctuate because of water retention rather than fat gain. Hard workouts salty meals hormonal changes and changes in carbohydrate intake can temporarily increase the amount of water your body holds. This can hide fat loss for several days so look at your average weight over a few weeks instead of focusing on one measurement.
You may not be giving your body enough time to show results. Weight loss does not happen at the same speed every week and short periods of unchanged weight are common. If you have only been doing cardio for a week or two keep your routine consistent and monitor your progress over several weeks before deciding that your plan is not working.
You might be sitting for most of the day outside your gym sessions. Even if you complete thirty minutes of cardio daily your total activity can still be low if you spend the rest of the day sitting at a desk or relaxing on the sofa. Adding regular walks taking the stairs and standing up throughout the day can increase your overall energy expenditure.
Stress could be another factor affecting your progress. When life gets stressful you may snack more often sleep poorly or find it difficult to follow your usual eating routine. Stress can also cause temporary water retention. Cardio can help with general well being but combining exercise with relaxation and consistent eating habits may make your weight management routine easier.
You could be focusing too much on the number of calories burned during each workout instead of your total daily habits. A great cardio session is useful but weight management also depends on meals snacks sleep and everyday movement. Instead of trying to burn as many calories as possible in the gym build a routine that you can follow comfortably for months.